Understanding combined heat and power, also known as cogeneration

What is combined heat and power (CHP)?

The first step to understanding CHP is to know what it is not. The typical U.S. power plant is only about 33 percent efficient, using three units of fuel to produce one unit of electricity. The rest gets turned into waste energy, mainly heat that’s vented into the atmosphere. Most plants can’t recycle this heat because they’re located remotely, far from consumers, and heat cannot travel far before turning cold. This kind of energy production—called “central” generation—is the dominant way of making power in the U.S.

Combined heat and power turns these numbers on their head, providing what the U.S. Environmental Protection Agency (EPA) calls “an efficient, clean, and reliable approach to generating electricity and heat energy from a single fuel source.” The key is that cogeneration plants generate energy on site at manufacturing facilities and other large institutions. That enables these plants to recycle their waste heat into clean electricity and useful steam, which can be used to warm nearby buildings or to assist various industrial processes.

Instead of throwing away two-thirds of the energy, combined heat and power plants utilize two-thirds or more of the energy they have at their disposal. Essentially, they do two jobs (generating heat and generating electricity) with one fire.

“By installing a CHP system designed to meet the thermal and electrical base loads of a facility,” the EPA says, “CHP can greatly increase the facility’s operational efficiency and decrease energy costs.”

What is waste heat recovery?

Waste heat recovery is combined heat and power’s sister technology. Like combined heat and power, it turns excess heat into clean electricity and useful steam. The difference is that it captures the waste heat a manufacturer is already emitting rather than providing all of the energy from scratch.

Here’s how it works. A “waste heat recovery boiler” contains a series of fluid-filled tubes placed throughout the area where heat is released. When high-temperature heat meets those tubes, a vapor (traditionally steam) is produced, which in turn powers a turbine that creates electricity. This process is similar to that of other fired boilers, but in this case, waste heat replaces a traditional flame as the initial source of energy. No fossil fuels are used in this process. Metals, glass, pulp and paper, silicon and other production plants are typical locations where waste heat recovery can be effective.

What is the potential for energy recycling?

A 2007 Department of Energy study found untapped potential for 135,000 megawatts of combined heat and power in the U.S. Meanwhile, a Lawrence Berkley National Laboratory study identified another 64,000 megawatts that could be obtained from industrial waste energy recycling, not counting CHP. Together, these two forms of energy recycling could provide 40 percent of total U.S. electricity needs.

Widespread use of energy recycling could cut U.S. greenhouse gas pollution by an estimated 20 percent. As of 2005, about 42 percent of U.S. emissions came from the production of electricity and 27 percent from the production of heat. Achieving greater efficiency in these areas is thus crucial to curbing climate change.